1. Field of the Invention
This invention relates to a thin-film-sandwich type optical reading device having an electrode of a transparent electrically conductive film on one surface of a photoconductive film and a metal electrode on the other surface of it, the optical reading device being used as, for example, a photo-sensor array in a facsimile transmitter, and so forth. The present invention is also concerned with a method for manufacturing such optical reading device.
2. Description of Prior Art
FIGS. 3, 4(a), and 4(b) of the accompanying drawings illustrate a structure of a conventional sandwich type optical reading device, in which FIG. 3 is a plan view of the optical reader, FIG. 4(a) is a cross-sectional view taken along a line IV A--IV A in FIG. 3, and FIG. 4(b) is a partial cross-sectional view taken along a line IV B--IV B in FIG. 3.
The optical reader as illustrated in these figures of drawings is constructed with a plurality of metal electrodes 2 disposed on one surface of an insulating base plate 1 in a spaced apart relationship for each bit, a photoconductive film 3 in a rectangular shape made of amorphous silicon, and a transparent electrically conductive film 4 also in a rectangular shape.
While the above-mentioned photoconductive film 3 continuously covers the metal electrodes 2 without being spaced apart for each bit, hence it can be easily manufactured, it has a disadvantage such that there would unavoidably take place cross-talk among the bits, even if it is formed of a highly resistive, undoped amorphous silicon, with the consequence that no reading can be done with high resolving power. It has also such a point of problem that a region where the photo-current occurs is not limited only to an overlapped portion of the metal electrode 2 and the transparent electrically conductive film 4, but it is dependent on the area of the photoconductive film 3 with the consequent expansion of its sensitivity region.
As one way of solving such points of problem with the conventional optical reader having the rectangular photoconductive film, there has been a practice of dividing the photoconductive film into each bit, which takes an example after the solar battery. One example of such structure is shown in FIGS. 5(a) and 5(b) of the accompanying drawings, in which FIG. 5(a) is a cross-sectional view taken along a line VA--VA in FIG. 5(b) and FIG. 5(b) is a cross-sectional view taken along a line V--V in FIG. 5(a).
As illustrated, this type of the optical reader is so constructed that a plurality of metal electrodes 2 which are spaced apart for each bit are disposed on one surface of the insulating base plate 1, and each of n-, i-, and p-type amorphous silicon films is formed as the photoconductive film 3 on each of the metal electrodes 2 in the sequence of n-i-p with perfect isolation among the bits. In addition, with a view to reducing leak current from the end face between the transparent electrically conductive film 4 and the photoconductive film 3 on the metal electrode 3, the end face of the photoconductive film 3 is covered with an insulating film 5, after which the transparent electrically conductive film 4 is formed over these metal electrodes, the photoconductive film, and the insulating film.
While this type of optical reader, wherein the photoconductive film 3 is perfectly spaced apart among the bits, is free from the undesirable cross-talk among the bits, hence reading can be done with high resolving power, it still has shortcomings such that the region where the photo-current occurs is not limited only to the overlapped portion of the metal electrode 2 and the transparent electrically conductive film 4, but it is dependent on the area of the photoconductive film 3, and further that the number of working steps increases to make its manufacture complicated and troublesome.
As described in the foregoing, the conventional optical reading device having the rectangular photoconductive film suffers from its disadvantages such that the cross-talk among the bits is unavoidable, and the photo-current is dependent on the area of the photoconductive film with the consequence that its photosensitive region tends to expand. Moreover, the optical reading device having the photoconductive film perfectly spaced apart among the bits also suffers from its disadvantages such that the photo-current is dependent on the area of the photoconductive film with the result that its photosensitive region tends to expand, and that the number of manufacturing steps increases to make its manufacture complicated and troublesome.
As the result of strenuous studies and researches made by the present inventors on the cause of the above-mentioned points of problem, it has been discovered that, when the photoconductive film is not perfectly spaced apart for each bit, the cross-talk would take place as soon as the adjacent bits become closer to a distance of from 50 to 100 .mu.m or so. Further, change in the photo-current by the area of the photoconductive film is found to occur in the following situations: that is to say, in case the transparent electrically conductive film is made an electrode common to every bit and is grounded, (1) a portion where the metal electrode and the photoconductive film overlap each other without presence of the transparent electrically conductive film has the photosensitivity; on the contrary, (2) a portion where the photoconductive film and the transparent electrically conductive film overlap each other without presence of the metal electrode has the photosensitivity which is substantially negligible.